1. Field of the Invention
The teachings herein are directed to a system for distributing bone graft material in an intervertebral disc space.
2. Description of the Related Art
Bone grafts are used in spinal fusion, for example, which is a technique used to stabilize the spinal bones, or vertebrae, and a goal is to create a solid bridge of bone between two or more vertebrae. The fusion process includes “arthrodesis”, which can be thought of as the mending or welding together of two bones in a spinal joint space, much like a broken arm or leg healing in a cast. Spinal fusion may be recommended for a variety of conditions that might include, for example, a spondylolisthesis, a degenerative disc disease, a recurrent disc herniation, or perhaps to correct a prior surgery.
Bone graft material is introduced for fusion and a fusion cage can be inserted to help support the disc space during the fusion process. In fact, fusion cages are frequently used in such procedures to support and stabilize the disc space until bone graft unites the bone of the opposing vertebral endplates in the disc space. A transforaminal lumbar interbody fusion (TLIF), for example, involves placement of posterior instrumentation (screws and rods) into the spine, and the fusion cage loaded with bone graft can be inserted into the disc space. Bone graft material can be pre-packed in the disc space or packed after the cage is inserted. TLIF can be used to facilitate stability in the front and back parts of the lumbar spine promoting interbody fusion in the anterior portion of the spine. Fusion in this region can be beneficial, because the anterior interbody space includes an increased area for bone to heal, as well as to handle increased forces that are distributed through this area.
Unfortunately, therein lies a problem solved by the teachings provided herein. Currently available systems can be problematic in that the methods of introducing the fusion cage and bone graft material leaves pockets in regions of the intervertebral space that are not filled with bone graft material, regions in which fusion is desired for structural support. These pockets can create a premature failure of the fused intervertebral space due to forces that are distributed through the regions containing the pockets, for example, when the patient stands and walks.
Traditional fusion cages, such as the Medtronic CAPSTONE cage, are designed to be oversized relative to the disc space to distract the disc space as the entire cage is inserted. However, this makes it difficult to insert and position properly. In response to the problem, the art has developed a number of new fusion cages, such as the Globus CALIBER cage which can be inserted at a low height and expanded vertically to distract the disc space. Unfortunately, these types of devices have the typical graft distribution problem discussed above, in that they do not provide a path for bone graft to be inserted and fill in the space surrounding the cage or within the cage. They have other problems as well, including that the annulotomy must be large to accommodate a large enough cage for stability, and this large opening necessitates more trauma to the patient. Moreover, they can also create the additional problem of “backout”, in that they cannot expand laterally beyond the annulotomy to increase the lateral footprint of the cage relative to lateral dimension of the annulotomy. Since it takes several months for the fusion to occur to completion in a patient, the devices have plenty of time to work their way out of the space through the large annulotomy.
Accordingly, and for at least the above reasons, those of skill in the art will appreciate bone graft distribution systems that facilitate an improved distribution of graft material throughout the intervertebral space. Such systems are provided herein, the systems configured to (i) effectively distribute bone graft material both from the system, and around the system, to improve the strength and integrity of a fusion; (ii) reduce or eliminate the problem of failures resulting from a poor bone graft distribution; (iii) have a small maximum dimension in a collapsed state for a low-profile insertion into the annulus in a minimally-invasive manner, whether using only a unilateral approach or a bilateral approach; (iv) laterally expand within the intervertebral space to avoid backout of the system through the annulotomy; (v) vertically expand for distraction of the intervertebral space; and, (vi) provide an expansion in the intervertebral space without contracting the system in length to maintain a large footprint and an anterior position adjacent to the inner, anterior annulus wall, distributing load over a larger area, anteriorly, against the endplates.